The present invention relates to a liquid crystal shutter that uses a liquid crystal panel as a shutter. More particularly, the invention relates to a liquid crystal shutter, suitable for use in a printer, that uses a deformed helical structure ferroelectric liquid crystal panel, V-shaped antiferroelectric liquid crystal panel, or monostable ferroelectric liquid crystal panel as a shutter.
A printer technology that uses a liquid crystal panel as a shutter for controlling light transmission by opening or closing in accordance with a data signal, and that forms an image corresponding to the data signal by projecting transmitted light onto a photoconductor is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-227268 (by Sony).
FIG. 1 shows the operating principle of such a printer. The illuminating light source 1 comprises three primary colors of red (R), green (G), and blue (B) which are illuminated sequentially in a prescribed cycle. Illuminating light 2 is projected through a lens 3, and passed through or blocked by a liquid crystal shutter constructed from a liquid crystal panel; synchronized to the illuminating time of each of the R, G, and B colors, the shutter is opened or closed for a duration of time corresponding to image data. The liquid crystal shutter contains pixels corresponding to one line of data that a data signal outputs, and the light passed through the liquid crystal shutter is shone onto a photoconductor 5, i.e., photographic paper, to write one line of image data thereon. When the light of R, G, and B has been projected as described above, the photoconductor 5 is moved by one line, and light corresponding to the next line of data is projected. By writing image data line by line in this manner, an entire image is written onto the photographic paper 5. The time required to write the entire image is dependent on the time required to write one line, that is, the open/close speed of the liquid crystal shutter and the amount of the illuminating light 2. Reference character P indicates the moving direction of the photographic paper, i.e., the photoconductor.
Traditionally, liquid crystal materials such as STN and TN have been employed for liquid crystal shutters used in such printers.
The time required to write one image is dependent on the response speed of the liquid crystal shutter. When the conventional TN or STN liquid crystal element is used for the shutter for such a printer, it is difficult to raise the writing speed up to 1 ms, and it is therefore not possible to meet the market demand for faster image writing speeds. In view of this, deformed helical structure ferroelectric liquid crystal (hereinafter, referred to as xe2x80x9cDHF liquid crystalxe2x80x9d), V-shaped antiferroelectric liquid crystal, and monostable ferroelectric liquid crystal materials have been studied as new liquid crystal modes that can address such market demand.
The advantages that the above three modes have in common are that they exhibit fast response to pulse application, and that they provide the capability to display grayscale. However, since none of them has a memory capability, it has not been possible to use them for matrix liquid crystal displays other than active matrix displays that require the use of a switching device such as a TFT or MIM device at each pixel position.
On the other hand, in applications such as liquid crystal shutters for printers, since it is only necessary to control light for one line, the above-listed liquid crystal materials can be used without requiring the use of active devices. Furthermore, since they have no memory capability, they have the advantage that the liquid crystal can be driven using a simple waveform because the state of the liquid crystal can be controlled by just applying a pulse.
The DHF liquid crystal is thresholdless. The light transmittance of a DHF liquid crystal panel changes even when a very small amount of voltage is applied. Accordingly, the transmitting/non-transmitting state of the liquid crystal panel is determined based on whether or not a voltage is applied to an electrode. As a result, the active matrix type that directly drives each electrode has traditionally been employed for a liquid crystal panel that uses this type of liquid crystal. The inventor, however, has discovered a new driving method ideally suited for driving such a liquid crystal panel as a liquid crystal shutter using a pixel electrode and a counter electrode.
Each pixel is located at an intersection between the pixel electrode and the counter electrode, and the sum of the voltages applied to the pixel electrode and counter electrode is applied to the pixel. A write period is provided as a period for determining the transmittance (the amount of light transmission) of the pixel, and the voltage applied to the counter electrode is held constant at or near 0 V so that the amount of light transmission of the pixel is dependent only on the voltage applied to the pixel electrode. Further, the voltage to be applied to the pixel electrode is produced as a single pulse or a plurality of pulses and, to eliminate polarity skewness, each write period is divided into two subfields and the polarity of the single pulse or the plurality of pulses is reversed between the two subfields.
In the DHF liquid crystal panel, the amount of light transmission is controlled by utilizing the distortion of the helical structure of the liquid crystal. It is therefore desirable that the degree of the helical structure distortion be kept constant at all times so that the amount of light transmission can be controlled accurately. To achieve this, it becomes necessary to place the distortion of the helical structure in the same state each time a single pulse is applied to the pixel. Here, if the sum of the voltages applied to the pixel electrode and counter electrode is set to 0 V before applying a single pulse, since no voltage is applied within the DHF liquid crystal panel, the helical structure of the DHF liquid crystal stabilizes. That is, if the sum voltage is held constant at or near 0 V during each subfield period except the period that the single pulse is applied, the helical structure stabilizes during that subfield period, making it easier to control the amount of light transmission by the single pulse.
Further, for stabilization of the helical structure, it is preferable to increase the period during which the sum voltage is held constant at or near 0 V. To increase this period, it is desirable that the single pulse to be applied to the pixel electrode be always placed at the beginning or at the end of the subfield. This serves to increase the time interval between the application of one single pulse and the application of the next single pulse.
To ensure stabilization of the helical structure, a reset period during which the sum voltage is held constant at or near 0 V may be provided before or after the subfield. The provision of this reset period is particularly effective when the single pulse is not always placed at the beginning or at the end of the subfield.
It has also been discovered that the degree of the helical structure distortion for the magnitude of the applied voltage varies depending on the temperature of the DHF liquid crystal. Accordingly, for the optimum amount of light transmission, the width or height of the single pulse must be varied according to the temperature of the DHF liquid crystal panel. As the temperature of the DHF liquid crystal panel lowers, the degree of distortion of the DHF liquid crystal for the magnitude of the applied voltage decreases, and therefore the width or height of the single pulse should be increased. On the other hand, as the temperature of the DHF liquid crystal panel rises, since the degree of distortion of the DHF liquid crystal for the magnitude of the applied voltage increases, the width or height of the single pulse should be reduced.
The V-shaped antiferroelectric liquid crystal is also thresholdless. The light transmittance of a V-shaped antiferroelectric liquid crystal panel changes even when a very small amount of voltage is applied. Accordingly, the transmitting/non-transmitting state of the liquid crystal panel is determined based on whether or not a voltage is applied to an electrode. As in the case of the DHF liquid crystal panel, the inventor has discovered a new driving method ideally suited for driving such a V-shaped antiferroelectric liquid crystal panel as a liquid crystal shutter using a pixel electrode and a counter electrode.
On the other hand, the monostable ferroelectric liquid crystal has a threshold, and is placed in a transmitting state when a voltage of either positive or negative polarity is applied. Within a certain voltage range, the light transmittance of a monostable ferroelectric liquid crystal panel changes even when a very small amount of voltage is applied. As in the case of the DHF liquid crystal panel, the inventor has discovered a new driving method ideally suited for driving such a monostable ferroelectric liquid crystal panel as a liquid crystal shutter using a pixel electrode and a counter electrode.
More specifically, the liquid crystal shutter of the present invention can be summarized as follows.
The liquid crystal shutter of the present invention uses as a shutter a liquid crystal panel constructed by sandwiching a DHF liquid crystal between a pair of substrates provided with a pixel electrode and a counter electrode. The driving waveform for the panel has a write period consisting essentially of two subfields, in each of which a single pulse is applied to a pixel. The single pulse applied in one subfield is opposite in polarity to the single pulse applied in the other subfield. On the other hand, a constant voltage of 0 V or close to 0 v is applied to the pixel during a period in which neither of the single pulses is applied.
Alternatively, the liquid crystal shutter of the invention uses as a shutter a liquid crystal panel constructed by sandwiching a V-shaped antiferroelectric liquid crystal between a pair of substrates provided with a pixel electrode and a counter electrode. The driving waveform for the panel has a write period consisting essentially of two subfields, in each of which a single pulse is applied to a pixel. The single pulse applied in one subfield is opposite in polarity to the single pulse applied in the other subfield. On the other hand, a constant voltage of 0 V or close to 0 V is applied to the pixel during a period in which neither of the single pulses is applied.
In the V-shaped antiferroelectric liquid crystal panel, a pulse smaller in width and opposite in polarity to the single pulse may be applied immediately following each single pulse.
Alternatively, the liquid crystal shutter of the invention uses as a shutter a liquid crystal panel constructed by sandwiching a monostable ferroelectric liquid crystal between a pair of substrates provided with a pixel electrode and a counter electrode. The driving waveform for the panel has a write period consisting essentially of two subfields, in which a first single pulse for making a pixel transparent to light is applied at the end of the first subfield and a second single pulse, opposite in polarity to the first single pulse, for not making the pixel transparent to light is applied at the beginning of the second subfield. On the other hand, a constant voltage of 0 V or close to 0 V is applied to the pixel during a period in which neither of the single pulses is applied.
In the monostable ferroelectric liquid crystal panel, a pair of pulses consisting of a first single pulse for making the pixel transparent to light and a second single pulse, opposite in polarity to the first single pulse, for not making the pixel transparent to light may be applied in each of the subfields.
In each of the above-described liquid crystal shutters, the pixel electrode consists of a plurality of electrodes, and the counter electrode consists of one electrode. In this case, a single pulse is applied to the pixel electrode, a constant voltage of 0 V or close to 0 V is applied to the counter electrode, and a sum voltage representing the sum of the voltages applied to the both electrodes is applied to the pixel.
The single pulse is applied at the beginning or at the end of each of the subfields, and the amount of light transmission of the liquid crystal panel is controlled by controlling the width or height of the single pulse.
Instead of the single pulse, one or more than one pulse may be applied to the pixel in each subfield, and the amount of light transmission of the liquid crystal panel is controlled by controlling the number of pulses applied.
A reset period during which the sum voltage applied to the pixel is held constant at or near 0 V may be provided immediately preceding the write period.
The liquid crystal panel is provided with a temperature sensor and a device for varying the width or height of the single pulse in accordance with a change in the temperature of the liquid crystal panel. This device increases the width or height of the single pulse when the temperature of the liquid crystal panel lowers, and reduces the width or height of the single pulse when the temperature of the liquid crystal panel rises.
Similarly, the liquid crystal panel is provided with a device for varying the number of pulses in accordance with a change in the temperature of the liquid crystal panel, and the device increases the number of pulses when the temperature of the liquid crystal panel lowers, and reduces the number of pulses when the temperature of the liquid crystal panel rises.
Further, the liquid crystal shutter of the invention can be used in a printer that is equipped with a liquid crystal shutter using a liquid crystal panel as a shutter for controlling light transmission by opening or closing in accordance with a data signal, and that forms an image corresponding to the data signal by projecting transmitted light onto a photoconductor.